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Protein S Measurement
A protein S measurement is a test used to measure the amount of protein S in your body. This protein plays a role in controlling the blood clot...

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What is a Protein S Measurement?

Protein S is one of the many vital proteins in the human body. It works to control the blood clotting process. The blood’s ability to clot is very important. It helps prevent excessive blood loss when an injury occurs. However, a blood clot in an artery or a vein (called thrombosis) can be extremely dangerous.

The body contains coagulants and anticoagulants. Coagulants encourage clotting, while anticoagulants help prevent it. Protein S is an anticoagulant. If there is not enough of it, it increases the chances that a harmful type of blood clot will form. The correct amount of protein S is needed to ensure the blood clotting process functions properly.

A doctor will usually order a protein S measurement to find out whether the protein is present in the right levels.

Why a Protein S Measurement Is Performed

The most common reasons that a doctor may order a protein S measurement are:

Family History

In some instances, a protein S deficiency is inherited. Some people are simply born with a shortage of this particular anticoagulant. A doctor may order testing if a patient has one or more close family members with a history of dangerous blood clots or a known deficiency of protein S.

Blood Clot Incidents

If a person develops a blood clot in a vein or an artery (thrombosis), a protein S measurement will often be performed. This can help doctors determine the cause of the thrombosis. Clots associated with a lack of protein S tend to form in veins.

Symptoms and Common Causes of Protein S Deficiency

Some causes of low protein S levels include:

  • genetics
  • pregnancy
  • HIV
  • taking prescription anticoagulants such as warfarin and some other types of medicines
  • liver disease
  • vitamin K deficiency
  • infection


For most people with a protein S deficiency, a potentially dangerous blood clot is often the first sign that something is wrong. The clot most often appears in the leg or chest area and there are usually no symptoms leading up to the event.

It is also important to note that a protein S deficiency does not always mean a person will develop thrombosis. People affected may go through their entire lives without any problems.

How Is a Protein S Measurement Done?

Your doctor will evaluate your medical history and your medication use before the test to decide when the test should be done and if you need to do anything to prepare. Two considerations before testing include:

  • The test should not be done until at least 10 days after a thrombosis event.
  • You will need to stop taking anticoagulants for a minimum of two weeks to ensure accurate results.

You will need to provide a blood sample for a protein S measurement. A needle will be inserted into a vein and blood will be collected in a vial. You may experience some minor pain as the needle is being inserted and some soreness afterward. Serious complications, however, are rare.

Interpreting the Measurement Results

Your doctor will interpret your results and discuss with you any diagnosis or abnormalities, if present. Results are usually presented in terms of percent inhibition. These percentage values should usually fall between about 60 and 150. There might be some slight differences among testing facilities. High levels of protein S are not typically cause for concern, whereas low levels indicate a deficiency in protein S, which could signal an infection or a likelihood of blood clots.

Following Up After a Confirmed Protein S Deficiency

If a protein S deficiency does exist, the follow-up will depend on the cause. Sometimes, there is another condition causing protein S levels to be lower than they should be. In these cases, addressing the underlying condition is the logical next step.

For those with an inherited deficiency, the focus will usually be on reducing or eliminating risk factors for clots. Simple lifestyle changes such as quitting smoking, exercising often, and avoiding the use of birth control pills are just some ways to lessen the chances that a lower than optimal amount of protein S will lead to a potentially dangerous clot.

Written by: Krista O'Connell
Edited by:
Medically Reviewed by:
Published: Aug 15, 2012
Published By: Healthline Networks, Inc.
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